2.1. SIGNAL TRANSDUCTION
Cellular signal transduction is a fundamental mechanism whereby external stimuli that regulate diverse cellular processes are relayed to the interior of cells. The process is generally initiated by the binding of extracellular factors (such as hormones and growth factors) to membrane receptors on the cell surface. The biochemical pathways through which signals are transmitted within cells comprise a circuitry of directly or functionally connected interactive proteins. Each protein component in a pathway integrates signals from upstream activators and passes them onto various downstream effector proteins.
One of the key biochemical mechanisms of signal transduction involves the reversible phosphorylation of tyrosine residues on proteins. The phosphorylation state of a protein may affect its conformation and/or enzymic activity as well as its cellular location. The phosphorylation state of a protein is modified through the reciprocal actions of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Generally, the level of tyrosine phosphorylation increases after the cell has been stimulated by an extracellular factor. Research in this area has largely focused on protein tyrosine kinases (Sefton et al., 1980 Cell 20:807-16; Heldin & Westermark, 1984 Cell 37:9-20; Yarden and Ullrich, 1988 Ann. Rev. Biochem. 57:443-78; Ullrich and Schlessinger, 1990 Cell, 61:203-12).
Protein tyrosine kinases comprise a large family of transmembrane as well as cytoplasmic enzymes with multiple functional domains (Taylor et al., 1992 Ann. Rev. Cell Biol. 8:429-62). The binding of an extracellular factor or ligand allosterically transduces a signal to the inner face of the cell membrane where the cytoplasmic portion of the receptor protein tyrosine kinase (RPTKs) initiates a cascade of molecular interactions that disseminate the signal throughout the cell and into the nucleus.
Ligand-induced activation of the kinase domain and its signalling potential are mediated by receptor dimerization. Receptor dimerization stabilizes the interactions between adjacent cytoplasmic domains, and activates the intrinsic kinase activity of the receptor. Once activated, the receptor self-phosphorylates (autophosphorylation or transphosphorylation) on specific tyrosine residues in the cytoplasmic domain (Schlessinger, 1988, Trends Biochem. Sci. 13:443-7, Schlessinger and Ullrich, 1992, Neuron, 9:383-91, and references therein). In case of insulin receptor-type RPTKs, the receptor exists naturally as a dimer, undergoing a conformational change and autophosphorylation upon ligand binding.
While it is widely appreciated that these RPTKs assume a key role in signal transduction, the part played by phosphatases remains poorly understood. Like the PTKs, the protein tyrosine phosphatases comprise a family of transmembrane and cytoplasmic enzymes. (Hunter, 1989, Cell 58:1013-16; Fischer et al., 1991, Science 253:401-6; Saito & Streuli, 1991, Cell Growth and Differentiation 2:59-65; Pot and Dixon, 1992, Biochem. Biophys. Acta, 1136:35-43). It is believed that RPTKs play a triggering role in signal transduction, while RPTPs guarantee that the trigger is reset, thereby serving to deactivate the pathway. However, certain kinases may provide inhibitory functions by phosphorylation of inhibitor sites on a signaling molecule, and certain phosphatases may have triggering functions by dephosphorylating the inhibitory sites. The first PTP purified was a cytoplasmic (nonreceptor) PTP (CPTP), PTP1B (Tonks et al., 1988, J. Biol. Chem. 263:6722-30) which unexpectedly shared sequence similarity with the cytoplasmic domain of a leucocyte surface antigen, CD45. Subsequently, CD45 was shown to possess tyrosine phosphatase activity and was recognized as a receptor-type PTP (RPTP) (Tonks et al., 1988 Biochemistry 27:8696-701).
While mammalian RPTPs and CPTPs share a homologous core catalytic domain, diverse noncatalytic sequences have also been observed. Some RPTPs contain Ig-like and/or fibronectin type III repeats in their extracellular portions (e.g., LAR, Streuli et al., 1988, J. Exp. Med. 168:1523), others have small extracellular glycosylated segments (e.g., RPTP.alpha., Sap et al., 1990, Proc. Natl. Acad. Sci. USA 87:6112; and RPTP.epsilon., Krueger et al., 1990, EMBO J 9:3241). In all cases, the putative ligands have yet to be identified. Other phosphotyrosine phosphatases such as PTP1B, PTP.mu., PTP1C, TC-PTP, PTPH1, RPTP.kappa., and CD45 have been cloned and their cDNAs are described in Chernoff et al., 1990, Proc. Natl. Acad. Sci. USA, 87:2735-9; Gebbink et al., 1991, FEBS Lett. 290:123-30; Shen et al., 1991, Nature, 352:736-9; Cool et al., 1989, Proc. Natl. Acad. Sci. USA., 86:5257-61; Gu et al., 1991, Proc. Natl. Acad. Sci. USA, 88:5867-71; Jiang et al., 1993, Mol. Cell Biol., 13:2942-51 and; Charbonneau et al., 1988, Proc. Natl. Acad. Sci. USA, 85:7182-6 respectively. Some PTPs and PTKs contain similar structural components. For example, members of both protein families may contain a homologous SH2 (src-homology 2) domain (reviewed in Koch et al., 1991, Science 252:668-74).
Although PTPs appear to be an integral part of the signal transduction mechanism, their specific functions have not been defined (Walton et al., 1993, Ann. Rev. Biochem. 62: 101-120).